The present invention relates to a plant for generating electric power comprising at least two generators separately driven by wind power and adapted to feed an alternating voltage to an alternating voltage line in common, a converter adapted to convert alternating voltage into direct voltage and conversely connected with its alternating voltage side to said alternating voltage line, a unit adapted to control the switchings of power semiconductor devices included in a converter and thereby the frequency of the alternating voltage on the alternating voltage line from the generators and an arrangement adapted to determine optimum frequency of the voltage on the alternating voltage line for the wind conditions prevailing, with respect to the maximum power that may be generated by the plant, and send orders to the unit to control the converter for obtaining this frequency of said alternating voltage, as well as a method for operation of such a plant according to the preamble of the appended independent method claim.
The invention is not restricted to any particular ranges with respect to the power and voltage levels.
The magnitude of the power delivered by an individual wind power generator depends upon the wind velocity and the frequency of the alternating voltage delivered by the generator. Thus, for a given wind velocity there is a maximum for the electric power that may be delivered by the individual wind power generator at a given frequency. This frequency increases with the wind velocity. The frequency of the alternating voltage line is determined through the control of the converter by the unit. Thus, the frequency of the alternating voltage received from the different wind power generators of the plant will be just as high.
A large amount of wind power generators may be included in a plant of this type, and these may together form a so called wind power park and be distributed over a land and/or water (usually sea) region of a not neglectible area. This means in its turn that the different wind power generators will operate with different local wind velocities, so that it will not be evident which frequency is to be chosen for delivering a maximum power on the alternating voltage line to the converter.
In a plant of the type defined in the introduction and already known it has been tried to solve this problem by providing said arrangement with members measuring the different local wind velocity, whereupon an estimate of a so called average wind velocity is made, and this then forms a basis for a calculation of the frequency in common to the alternating voltage of the wind power generators. However, it has turned out that this method is not as precise as desired, since such an average wind velocity is sometimes a too coarse measure of the optimum frequency with respect to the output power delivered. This means that not neglectible amounts of energy are then wasted for nothing.
The object of the present invention is to provide a plant and a method making it possible to increase the efficiency of a plant of the type defined in the introduction, so that a frequency of said alternating voltage resulting in a maximum total power output from the wind power generators may be selected as the basis for the control of the converter under the most varying wind conditions.
This object is according to the invention obtained by providing said arrangement in such a plant with means adapted to measure the total power fed from the wind power generators towards the converter and members adapted to send orders to the unit for adjusting the control of the converter depending upon power values delivered by said means so that a power maximum is obtained.
By delivering a value through measurement in this way of the total power delivered by the wind power generators together at a given location and adapt the control of the converter so that a maximum of the power measured is obtained the optimum frequency of the alternating voltage for each given wind condition may really be selected. It has turned out that the power that may be delivered by the plant may be increased by 5-15% with respect to the determination of an average in velocity already known through such a determination of a power maximum, which corresponds to 15-30% increase with respect to the case of a plant having a fixed predetermined frequency. Thus, the frequency may in this way perhaps not be exactly an optimum for any of the wind power generators, but it will be the best for their total ability to generate electric power. It is here pointed out that the patent claim definition xe2x80x9cmeasure the total power fed towards the converter from the wind power generatorsxe2x80x9d is not to be interpreted as a restriction of the invention to the case of a direct measurement of this power, but it is well possible that other quantities, such as voltage and current intensity, are measured and the power is determined on the basis thereof, and it may then be said that the power is measured indirectly.
According to a preferred embodiment of the invention said members of the arrangement are adapted to compare power values delivered by said means at different frequencies of the alternating voltage and elaborate orders to said unit for controlling the converter on the basis of the result of this comparison. The frequency for a power maximum may under given conditions comparatively easily be determined by proceeding in this way.
According to another preferred embodiment of the invention said members of the arrangement are adapted to send orders to the unit to change the frequency of the alternating voltage in a first direction, in an increasing or decreasing direction, and if the power measured by said means then increases send orders to continue the change of the frequency in said first direction, and if the power from a measurement carried out by said means decreases to the measurement following thereupon change the direction into an opposite second direction for the change of the frequency of the alternating voltage. The power maximum asked for may in this way be reliably determined. It may then be advantageous that the intervals of the changes of the frequencies between two subsequent measurements is made finer when a power maximum has been passed, i.e. when suddenly the power delivered decreases again. The regulation may be carried out intermittently, preferably at uniform time intervals, or it may proceed continuously for ensuring that the plant all the time operates at the maximum power that may be delivered.
According to another preferred embodiment of the invention the plant also comprises members adapted to detect the wind velocity close to at least one of the wind power generators and that said members of the arrangement are adapted to elaborate a start-control order for the control of the converter by the unit on the basis of the wind velocity detected corresponding to an optimum frequency of the alternating voltage calculated on the basis of the wind velocity. An arrival to the frequency lying close to said optimum frequency when starting the operation of a plant after an operation interruption, various disturbances of the regulation or the like, may be rapidly obtained by proceeding in this way, so that no power will be unnecessarily lost.
According to another preferred embodiment of the invention said means comprise members for measuring the current intensity and members for measuring the voltage on the alternating voltage line and members for calculating the power fed towards the converter from the. result of these measurements. This constitutes a way to achieve the power measurement according to the invention, which may be realized easily and thereby will be reliable and favourable from the cost point of view.
According to another preferred embodiment of the invention the plant comprises a transformer arranged along said alternating voltage line between the wind power generators and the converter for changing the level of the voltage arriving to the converter. Such a transformer enables the plant to deliver electric power having levels on the direct voltage side of the converter differing substantially from the voltage levels that may be delivered by the wind power generators. Thus, it may in this way for example be ensured that voltages advantageously high with respect to losses for power transmission over longer distances, maybe of several hundreds of kilovolts, may be obtained on the direct voltage side of the converter.
According to another preferred embodiment of the invention the converter is a so-called VSC-converter (Voltage Source Converter). This constitutes a particularly advantageous embodiment of the invention, since it is through such a converter possible to control the consumption of reactive power needed for the wind power generators. The direct voltage side of the converter is then preferably connected to a direct voltage network for transmitting electric power in the form of high voltage direct current (HVDC).
The invention also relates to a method for operation of a plant for generating electric power through at least two generators separately driven by wind power according to the appended independent method claim.
The advantages of such a method and of the preferred embodiments thereof defined in the dependent method claims appear as clear as desired from the discussion above of the different embodiments of the plant according to the invention.
Further advantages as well as advantageous features of the invention appear from the following description and the other dependent claims.